Automotive vehicles commonly include a parking brake which is operable to switch between an engaged state and a disengaged state. Somewhat recently, vehicles have been outfitted with electric parking brakes in which the parking brake is switched between the engaged and non-engaged states through operation of a motor. FIG. 1 schematically illustrates a known electric parking brake arrangement in which a single motor M is used in combination with one or more torque multiplication devices P1, P2 . . . Pn to achieve the desired output for operating the parking brake. The torque multiplication devices are typically in the form of belts, pulleys or a series of gears. The torque multiplication devices increase the torque produced by the motor output, but also reduce the speed.
FIG. 2 illustrates an example of a motor-operated parking brake, sometimes referred to as a motor-on-caliper parking brake. An actuator 12, which includes a motor, is operatively coupled to the brake 10. The caliper portion of the motor-on-caliper converts the rotational motion of the actuator into linear motion. FIG. 2B schematically illustrates a way in which this is accomplished. The actuator 12, under the operation of the motor, rotates a screw (lead screw) 16 which causes linear movement of a nut 18. The nut 18 pushes the caliper piston 20. A thrust bearing exists between the caliper and the screw, and allows the screw to rotate even though a relatively large load is being transmitted from the screw into the caliper. In a known manner, the movement of the piston linearly moves a brake pad toward and into contact with the brake rotor. Another brake pad opposes the one brake pad and contacts the opposite side of the brake rotor. The operation of the actuator 12, including the motor, thus produces a clamping force applied to the brake rotor.
Many known parking brakes utilize a single electric motor to effect operation of the parking brake. This motor must be relatively large to provide the power necessary to achieve the required brake performance. Motors of the size typically used exhibit a relatively low power density compared to smaller motors.
United States Application Publication No. 2003/0205437 proposes an electric brake assembly involving the use of two motors. FIG. 3 schematically illustrates the disclosed arrangement involving the use of spur gear trains P1, P2, P3 to produce an output. The drive shaft of one motor M1 engages and rotates the spur gear P1, while the drive shaft of the other motor M2 engages and rotates the spur gear P2. The two spur gears P1, P2 engage and rotate the third spur gear P3. The patent application publication states that the disclosed electric brake assembly permits a more compact design and allows two smaller-diameter motors, which exhibit lower inertia, to be used in place of the a larger-diameter single motor. The gear trains have only one input and one output, and so the speeds of the two motors are forced to be a constant ratio of one another.